Photocathodes

ABSTRACT

A photocathode comprising a ternary alloy having the general formula:

United States Patent 1191 Enstrom et al.

[ June 4,1974

1 'PHOTOCATHODES [75] Inventors: Ronald Edward Enstrom, Skillman;

Dennis Glendon Fisher, Titusville, both of NJ.

[73] Assignee: The United States of America as represented by the Secretary of the Air Force, Washington, DC.

[22] Filed: May 3, 1973 [21] Appl. No.: 357,037

Related U.S. Application Data [62] Division of Ser. No. 266,783,.1une 27, 1972.

OTHER PUBLICATIONS Shang, l.B.M. Tech. Discl. Bull; Vol. '13, N0. 11, April 1971, p. 3440. Shang, l.B.M. Tech. Discl. Bull.; Vol. 14, N0. 5. Oct. l97l,p. 1350. Antypas et al., Journal of Applied Physics. Vol. 42, No. 2, Feb. 1971, 580-586.

Primary Examiner-Martin H. Edlow Attorney, Agent, or Firm-Harry A. Herbert, .lr.; William .l. OBrien [57] 7 ABSTRACT A photocathode comprising a ternary alloy having the general formula:

wherein x is an integer of from 0.15 to 0.21. The cathode is selectively responsive to infra-red radiation,

' particularly at the 1.06 micron wavelength level.

5 Claims, 12 Drawing Figures PATENTEDJUN 41914 SHEET 8 [1F 7 1 PHOTOCATHODES BACKGROUND OF THE INVENTION- This application is a divisional application of US. Pat. Application Ser. No. 266,783 filed on June 27, 1972.

This invention relates to photocathodes and to a method for their manufacture. More particularly, this invention concerns itself with Gallium-Indium Arsenide ternaryalloys for use as negative electron affinity photocathodes sensitive in the infrared region to beyond 1.2 gm wavelengths. I

A number of materials are known to exhibit a photo- I emissive effect in that they emit electrons as a result of creased and the long wavelength limit raised when the surface of the photoemissive material is subjected to certain contaminations. This is particularly true when an alkali metal layer is superimposed on a metal surface. The greatest photoelectric responses are obtained, however, when the surface is a semiconductor rather than a pure metal. Most of the photoemitters having quantum efficiencies above 0.01 percent in the visible or near ultraviolet involve the use of alkali metals. Of these, cesium has given the best results. Most, if not all of the presently known photoemitters, in addition to their high quantum yield, exhibit selective photoemission. A photoemitter is said to be a selective emitter when the quantum efficiency, instead of rising monotonically with decreasing wavelength of radiation over a wide range of wavelengths, shows maxima and minima of quantum'efficiency.

' Two of the better known photosensitive surfaces are the cesium-antimony surface, and the cesium cesiumoxide silver surface. The first of these, namely, the cesium-antimony surface has a spectral response which lies chiefly in the blue. This type of surface, therefore, although having excellent white-light response, is not suitable where a photo-cathode sensitive to red light is required.

Depending upon the requirements of the photo-tube in which it is to be used, the cesium-antimony photocathode may take either of two forms. The first is the reflective-type having an opaque surface with the light falling on the same side from which electrons are emitted. The second is the transmission-type having a semitransparent surface with the exciting illumination falling on the back film through the supporting glass and the electrons emitted from the opposite side.

The other commonly used photosurface, the cesiated silver surface, is valuable because of its higher red response. This type of emitter is extremely sensitive in the red end of the visible spectrum, and its sensitivity extends well down into the near infrared. As was the case for the cesium-antimony photocathode, cesiated silver cathodes may be formed on opaque metal surfaces or as semi-transparent cathodes.

The property of selective photoemission exhibited by known photoemissive materials discussed heretofore has received considerable interest with the recent advent of high power lasers. These lasers emit at a wavelength of 1.06 pm and, at present, the only cathode available for detection of radiation in this spectral range is the well known cesiated silver AgO-Cs photocathode. Unfortunately, the quantum efiiciency of this photocathode at 1.06 p. is only about 0.0004 electrons/incident photon. However, with the present invention it has been found that efficiencies of about two percent at 1.06 p. (more than times higher than the cesiated silver surface) together with efficiencies at shorter wavelengths can be attained in the reflection mode with the photo-cathodes of this invention. With the transmission photocathodes of this invention, quantum yieldsof 1.2 percent at 1.06 11. have been achieved.

In attempting to find a highly efficient photoemissive material sensitive in the infra-red region, it was discovered that a gallium-indium-arsenide ternary alloy possessed the necessary photoemissive quantum efficiencies. These materials exhibit a quantum efficiency of from 1 to 2 percent as compared to an efficiency of 0.05 percent for the conventional cesiated silver cathodes. For optimum operating conditions, an alloy composition range of from about 17 percent to 21 percent indium-arsenide provided the highest efficiency of operation at the 1.06 1.1. range.

SUMMARY OF THE INVENTION In accordance with this invention, it has been found that a highly efficient negative electron affinity photocathode can be fabricated from a gallium-indiumarsenide ternary alloy. These cathodes exhibit an unexpected quantum efficiency in the 1.06;; wavelength range of about 2 percent forreflection-type cathodes and 1.25 percent for transmission-type cathodes. For highest efficiency of operation at 1.06 microns (1.17 eV photon energy), an optimum composition range for the alloy should comprise'from about 17 percent to 21 percent indium-arsenide. This narrow composition range corresponds very closely to the threshold composition of about 15 percent indium-arsenide.

Accordingly, the primary object of this invention is to develop a highly efficient photo-cathode characterized by having a photoemissive selectivity in the infrared region.

Another object of this invention is to provide a negative electron affinity photocathode that exhibits a quantum efficiency in excess of 1 percent at 1.06u wavelengths.

Still another object of this invention is to provide a gallium-indium-arsenide alloy for use as a photoemissive material sensitive inthe infrared range.

The above and 'still other objects and advantages of the present invention will become readily apparent upon consideration of the following detailed description thereof when taken in conjunction with the accompanying drawings.

DESCRIPTION OF THE DRAWINGS In the drawings:

FIG. 1 illustrates in schematic form an apparatus suitable for use in fabricating the photocathodes of this invention;

FIG. 2 is a graph indicating the infrared sensitivity of the alloys of this invention;

FIG. 3 is a graph showing the absorption coefficient of the alloys of this invention at 1.06 1.;

FIGS. 4 and 5 are graphs showing quantum yields of the alloys of this invention with FIG. 4 showing the expected yield while FIG. 5 shows the observed yield at 1.06m

FIG. 6 is a graph showing spectral yield curves for cathodes of varying band-gap showing band-gap limited emissions to about 1.311.;

FIG. 7 is a graph showing the alloy compositions of this invention as function of the HCl flow ratio to the indiumand gallium sources shown in FIG. 1;

FIG. 8 is a graph showing the energy gap for the undoped alloys of this invention as a function of compositron;

FIG. 9 is a schematic illustration of a UI-IV Vacuum System used for activating the photocathodes of this invention;

FIG. 10 is a graph showing the 1.06p. response versus the composition for alloys doped with a carrier concentration of l X IO /cm;

FIG. 11 is a graph showing the 1.0641. response versus the doping level for cathodes with X E 0.20; and

FIG. 12 illustrates in cross-section a photocathode device of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT The present invention encompases the fabrication of negative electron affinity photocathodes sensitive in the infrared wavelength region to beyond 1.2 microns wavelength. It has been found that Ga ,In,As ternary alloys, wherein x is an integer from about 0.05 to 0.75, exhibits the necessary selective photo-emission response and high quantum efiiciencies needed for use in the infra-red wavelength range to beyond 1.2.microns. Alloys in which x is an integer of from about 0.17 to 0.21 (-l7% 21% In As) possess an optimum composition range that produces the highest operational efficiency at the 1.06 micron infra-red wavelength range.

The relatively narrow optimum composition range of about 17 to 21 percent indium-arsenide shows an unexpected improvement in quantum efficiencies at the 1.06u level with the highest sensitivities being about two percent for reflection-type of photocathodes and about l.25 percent for transmission-type photocathodes. This constitutes an unexpected and significant improvement over the 0.05 percent quantum efiiciencies for the photocathodes of the prior art.

The gallium-indium-arsenide photocathodes of this invention are especially useful for incorporation into phototubes used in high performance laser illuminator systems and a low-light-level image pickup systems.

The general requirements for a photocathode for use at 1.06; include a Cs-O activated Ga-In-As alloy surface layer with a band gap on the order of 1 eV. Generally, the layer is suitably doped on the order of 10" holes/cm, and the trap concentrations and associated recombination losses should be minimal so that a long minority carrier diffusion length can be achieved. Thus, a method is required for growing suitable alloy materials with the necessary purityand band gaps. Conventional vapor-phase growth methods satisfy this requirement and the well known arsine method, as illustrated in FIG. I, has the ability to produce high quality material with high purity over a wide range of alloy composition. The Ga Jn As alloy system of the invention has been successfully activated with Cs--O layers in accordance with conventional procedures.

To further illustrate the invention reference is now made to the schematic illustration in FIG. 1 which shows an apparatus suitable for growing the Ga,.,. ln As photocathodes of this invention.

The growth apparatus is inherently simple, consisting essentially of a straight tube through which the pertinent gases flow. As shown schematically, Ga or In is transported as its subchloride formed by passing l-ICI gas over the molten metal. Arsenic is formed by the thermal decomposition of its hydride (Asl-l High purity palladium-diffused H is used as a carrier gas, and zinc vapor provides the p-type dopant to achieve hole concentrations from 1X10 to 4X10 cm As the gases pass over the substrate, they react with its surface to form an epitaxial deposit, the composition and electrical properties of which are established by the vapor constitution.

The alloys are grown on (100)-oriented singlecrystalline GaAs substrates having areas of l to 2 cm. For practical photocathode applications, the large-area surface is desirable. Prior to growth, the substrates are chemically polished in a solution containing H 80 H 0, and H O, in the ratio 4:]: l. l

The spectral response of the photocathode is determined by the bandgap of the semiconductor, and for response out to 1.4;; materials with bandgaps as low as 0.9 eV have been investigated. To achieve high quantum yields at long wavelengths, it has been found that the alloys must be homogeneous and have a very clean surface after growth, and the bandgap of the alloy and the acceptor concentration must be optimized. In addition, the activation procedure has to be sufficiently sophisticated so that negative electron affinity can be achieved on the surface of the low bandgap alloys.

It had been expected that as lnAs is added to GaAs to form Ga ,ln,As alloys, the broadband infrared sensitivity would increase with decreasing bandgap (increasing InAs content) as long as the negative electron affinity (NEA) condition is satisfied. This was expected because more of the infrared radiation in the standard tungsten lumen light is available for the generation of electrons in the photocathode alloy that can eventually be emitted into vacuum, as shown in FIG. 2 curve a. The response that was found is shown in FIG. 2, curve b.

Alloys containing greater than 15 percent lnAs can also be used for 1.06p. detectors. For these materials, FIG. 3 shows the 1.06p. absorption coefficient, as a function of lnAs composition. Since or increases with the percent lnAs, one would expect that the photoemission would correspondingly. increase as long as the NEA condition is fulfilled (i.e., for p semiconductors E, Work functions as low as 0.6 eV have been reported for alloys so there exists the possibility that 1.0611. photo sensitivity would increase with percent lnAs out to percent (E, z 0.6 eV) as illustrated in FIG. 4.

However, contrary to these expectations, it has been discovered experimentally that there exists a relatively narrow optimum composition range 17 percent 21 percent) very close to the threshold composition of 15 percent lnAs (corresponding bandgap of 1.17 eV) as shown in FIG. 5. Thus, for operation of photocathodes capable of highest efficiency of operation at 1.06m compositions within this range should be used.

Ga ,ln,As alloy samples in the band gap range 0.96 to 1.18 eV (0.3l x 0.14) were vapor grown using the arsine method referred to previously. A quartz growth tubeand separate metallic Ga and ln sources were used. Chemically polished GaAs substrates oriented 3 off the 100 direction were used for the growth of the alloy layers. The sample areas were l-2 cm During vapor growth, the layers were doped with acceptor impurities from a metallic Zn source to about 1X10 cm. The compositions were determined by x-ray diffraction, and this enabled a detennination of the band gap. Also, a photocathode comprising a thin gallium-indium-arsenide layer deposited onto the surface of a wider band-gap substrate than GaAs, such as gallium-indium-phosphide can be formed to give a photocathode with response into the visible region of the spectrum.

The large area surfaces were cleaned in Ul-lV by heating to temperatures near the decomposition point. They were subsequently activated with Cs-and O to produce peak infrared photosensitivity with a l/200 lumen source passed through'a Corning 7-56 filter. A

' molecular beam Cs source was used.

The amount of Cs on the activated sample for these ternary alloys was determined relative to that for GaAs by comparing the Cs exposure time for identical cesiation conditions. The amount of Cs required for the ternary layers was within about i 10 percent of that required for GaAs. Direct determination of the amount of Cs persent on GaAs cathodes has shown that the layer contains the equivalent of only about one monolayer of Cs O and one monolayer of Cs. Thus, the Cs 0 layer for the present narrow band gap alloys is also on the order of l monolayer thick. This result was confirmed by a comparison of the 0 exposure required to reach peak sensitivity for GaAs and the ternary alloys.

Following activation, spectral yield curves were obtained'. These curves are shown in FIG. 6, Samples 2 and 3 have the best 1.06 p. response, 1.7 percent and 1.6 percent, respectively. In all cases, the photoelectric threshold energy is approximately equal to the band gap, indicating that the negative electron affinity condition has been achieved. For sample No. 5, the yield near threshold is noticeably lower than that of the other samples. Nevertheless, the yield is considerably greater than can be expected from sources other than the valence band, such as direct emission from Cs O or from surface states. The yield increase at shorter wave lengths for this layer is thought to result from hot electron emission.

The Ga, ,ln,As alloys are prepared with separate Ga and In metal sources. The separate sources offer more flexibility in the control of the alloy composition so that the composition can be easily changed during growth, as is necessary for graded layers.

To prepare homogeneous alloys with the two metal sources, the MCI flows must be carefully controlled. As shown in FIG. 7, a linear relationship is found between the lnAs concentration and the HC m/HCl ratio. Therefore, the lnAs concentration can be easily adjusted over a wide range and excellent reproducibility at a given composition can be achieved. To assure complete reaction of the HO with the in metal source, es-.

' pecially at the higher l-lCl flow rates, a sufficient volume of In metal is used.

The growth rate is a function of alloy composition above 20 percent lnAs and varies from about 20 [L/l'lOUl at 20 percent to about 12 r/hour at 50 percent lnAs under our usual conditions of gas flow and furnce temperatures. However, by adjusting growth conditions, growth rates above 20 p./hour can be attained for alloys containing up to 50 percent lnAs.

The homogeneity of the alloy samples is good as evidenced by the sharpness of the x-ray powder diffraction lines. Graded or inhomogeneous layers would have diffuse diffraction lines. Electron-microprobe measurements confirm the good homogeneity of vapor-grown layers. For a 16.5 percent lnAs alloy, it was found that the composition is within i 1.0 percent of the average value over a distance of 1 cm on the surface, and identical composition values were found over distances of several mm. Good homogeneity is required to obtain long minority carrier lifetimes and diffusion lengths needed for high-yield photocathodes.

The alloys are very smooth and reflective and are comparable in appearance to GaAs layers prepared under good growth conditions.

The electrical properties of the alloys of the invention, as determined by Hall effect measurements, are shown in Table I.

Undoped GaAs samples contain about 5 l0 electrons/cm, and it may be seen that good mobility values have been achieved both at 300K and 77K. A further indication of the quality of our material is the high value of the mobility at 77K, 13,000 cm /V-sec, for an .n-type alloy containing 48% lnAs. For most of the photo-cathode samples, a hole concentration of about l l0 cm 3 is used. The room temperature mobility for p-type GaAs and Ga ,ln,As alloy layers is between 60 and cm lV-sec over the compositional range of interest. A companion GaAs sample to run with Example 2 in Table I was annealed at 900C for '15 minutes to annihilate dislocations. The resulting hole mobility values at 300K and 77K, were identical to those of the compansion sample, consistent with the fact that the majority carrier properties are not sensitive to dislocation density.

The photoemissive properties are strongly affected by the diffusion length of the photoexcited electrons. Briefly, it was found, using a scanning electron microscope, that the diffusion lengths for minority carriers (electrons) in GaAs, Ga ln As and Ga ln As layers doped to 1X10 holes/cm are 1.6g, 1.2g, and 08p, respectively. The value for GaAs is consistent with that determined from photoemissive yield data. Thus, we see that the minority carrier diffusion length is not strongly affected by alloying with lnAs and that alloy layers suitable for photocathodes have been prepared.

In the present invention Eg has been measured by extrapolating to zero the best straight line fit of the square of the optical density versus photon energy hv for undoped samples with compositions between and 50% InAs. The results are shown in FIG. 8. The composition was determined from measurements of the lattice parameters since Vegards law has been found to be valid for the GaAs-InAs alloy system. It should be noted that for alloys doped with Zn acceptor impurites, the energy gap would be about 0.03 eV lower.

The reflection-type photocathode materials of this invention were evaluated in one of three different types of vacuum systems. In each case the alloy surface was first cleaned by heating in vacuum with electron bombardment to temperatures near the decomposition point. The surface temperature was determined with an Ircon Model 300 optical thennometer. Following heating, the sample wax exposed alternately to Cs and 0 until a peak in the white-light sensitivity was reached. The 0', was obtained from a conventional silver oxygen leak," controlled by heating. The Cs source was either a molecular gun type (heated side arm containing an ampule of Cs) or a standard Cs-chromate channel, depending on the vacuum system.

Initially several medium-size glass systems with l/sec vac-ion pumps were used. In two of these systems, samples were mounted on a trolley so that several samples could be evaluated in a single pump down. The molecular gun type Cs source was used.

The best results were obtained in a Varian Model V-l2 demountable UI-IV system with a 500l/sec Vaclon pump. A schematic diagram is shown in FIG. 9. The twelve samples were mounted on a rotatable shaft and could be located at any of several different positions indicated. The samples could be installed into the system through one of the small side ports. The samples were heated at location 1. Following heating, the samples were activated with Cs and 0 at location 2. The Cs source is the molecular gun type. At this location, the response (including response with filters) can be measured during activation. Spectral response measurements were made at 3. A Jarrel Ash double monochromator with a tungsten-iodide light source was used. Its output was periodically measured with a calibrated phototube.

This system is especially well suited for sample evaluation for a number of reasons. (I) The environment is the same for all samples so that system effects are minimized when comparing one result to another. (2) It is possible to activate in the same way each time, i.e., Cs and O rates can be reproduced. (3) The pressure rise during heat cleaning is only slight 10 Torr) so that recontamination of the sample after cleaning is minimized. (4) Most important, several GaAs samples have been activated to sensitivities in the l500-l600 A/lm range, thus demonstrating that consistently good results can be obtained.

The effect of the vacuum system on sensitivity was most pronounced for the narrower band gap samples, the large V-l2 system giving considerably better results than the other two types. For example, using samples grown and activated under comparable conditions, white-light sensitivities in the 1500-1700 uA/lm range were obtained from GaAs in all three types of systems whereas white-light sensitivities differed by factors of 2 to 3 and 1.06 response differed by as much as a factor of 5 for samples with band gaps sufficiently narrow for 1.06 11. sensitivity. The higher sensitivities suggest that the cathode surface is cleaner due to better vacuum conditions and/or absense of contamination from glass in close proximity to the sample during heating. It is reasonable that surface cleanliness is more important for the case where the magnitude of the negative electron affinity is less, i.e., narrow band gap alloys. In any event, the V-l2 system was the most useful system for evaluating cathode parameters.

A variety of substrate materials can be considered for the preparation of reflection photocathodes, but, for transmission photocathodes substrates transparent to radiation of interest (e.g., 1.06 p.) must be used. For the latter, GaAs is the most practical substrate, and we have found that both undoped (n-lXl0 cm' and Cr-doped high resistivity (p-l0 ohm-cm) substrates have low absorption coefficients (2 to 5 cm") at 1.06 ;1.. High conductivity p-type substrates cannot be used for transmission photocathodes because of the very high free carrier absorption.

For reflection photocathodes, GaAs substrates doped with Zn, Si, and Cr have been examined. Each substrate has its advantages. Zn-doped substrates have the same lattice parameters as the initial vapor-grown layer which should minimize the introduction of imperfections, and consequently these have been most frequently used. Si-doped substrates are potentially more defect free, and Cr-doped high-resistivity substrates are convenient when making transmission measurements on the alloy layer.

It was found that Zn-doped substrates from different boules and other substrates with different acceptor impurities gave nearly the same sensitivites. For example, values as high as 920 y-a/lm for Zn-doped substrates and 870 uA/lm for Cr-doped substrates were measured for 14 percent samples prepared simultaneously. Similarly, 6 percent lnAs layers prepared simultaneously with Zn-doped and Si-doped substrates and activated in a seal-off type tube had nearly equivalent photoemissive properties, 500 and 430 uA/lm, respectively.

The electrical properties of lightly doped (-l0 /cm GaAs samples were substantially improved by reducing the growth chamber center zone temperature from l,040 to 875C. The effect of this change on photo-cathode response was investigated. Two 14 percent lnAs samples grown under the high and low temperature conditions were tested. The white-light sensitivities were 800 and 870 uA/lm, respectively. These quite comparable high sensitivities suggest that the center zone temperature does not influence the minority'carrier properties at relatively high doping levels (-l0 /cm) nearly so much as it affects the majority carrier properties at lower levels.

The normal method for preparing the surfaces of the alloy is heat treatment in vacuum at temperatures near the decomposition point. Surface preparation was also made by sputter-cleaning the alloy surfaces by argon ion bombardment. The post bombardment annealing temperatures required were comparable to temperatues normally used for heat cleaning. The white-light sensitivities for 17 percent InAs samples cleaned by heating only and by sputtering were about the same uA/lm).

' Tests were also performed to detennine whether the growth termination procedure produces a non-uniform surface composition. Two 14 percent lnAs samples from the same growth run were used. One of these was HCl vapor etched after growth. Upon subsequent activation, identical sensitivities of 860 uA/lm were obtained indicating that there is no significant gradation of homogeneity perpenducular to the surface in the asgrown layers.

The photosensitivity for alloys having an acceptor concentration of 1X10 cm are summarized in Table II. S and S are sensitivities to white light with no filter and with an infrared-passing Corning 7-56 filter, respectively; Y(l.06 p.) is the quantum yield in percent (electrons/incident photon).

TABLE ll Photosensitivities of Ga .,ln,As Alloys Example X l,(e SW 5,, Y(l.06 p) M). (I -All n) (uA/lm) (M The results tabulated above show several things. 1) For a given band gap, our activationprocedures result in reproducible broad-band photo-sensitivity. (2) Although a band gap of 1.17 eV is sufficiently narrow to result in 1.06 p response (1.06 1il .17 eV photon energy). the response is close to threshold and thus is low and not reproducible. (3) The decrease in white-light reaponse with decreasing E, is evidence of a decreasing surface escape probability which offsets the increase in uA/lm available (i.e., increased light absorption, especially at longer wavelengths). FIG. 10 shows explicitly the 1.06 micron response as a function of composition. There is a farily wide composition range between at least l7-2l percent lnAs where theresponse is virtually independent of composition. In this region, the increase in optical absorption at 1.06 pm with decreasing bandgap is presumably compensated by a decrease in surface escape probability. The existence of a range of relatively constant response suggests that the possibility of reproducible results from sample to sample should not be limited by compositional control 1 percent lnAs).

The doping concentration n, can affect the diffusion length L through both the lifetime and the mobility IL since Mobility decreases with increased doping because of increased scattering either at acceptor sites or at precipitates (imperfections consisting of large agglomerations of the dopant). Lifetime is generally degraded the bent band region. This gives a shorter distance over which the photoelectrons can lose energy which results in a smaller spread in energy for electrons arriving at the surface. For a given magnitude of negative electron affinity IXI there is a corresponding increase in the fraction which can escape as discussed earlier. Since both IXI and the band bending distance depend on band gap, the relative influence of doping on diffusion length and on surface escape probability also may be a function of band gap.

The influence of doping on 1.06 11. response for a band gap of 1.09 eV (20.5 percent lnAs) is shown in Table III.

TABLE III Photosensitivities of. Ga, In As Samples of Various Carrier Concentrations FIG. 11 shows the 1.06 a response as .a function of doping for,Ga ,,ln,As cathodes with x 0.20. The data suggests that 5X10 cm is a near optimum doping for a 20.5 percent composition.

Apparently the effect of n, on L is dominant over that of n,, on B for the dopings investigated.

With transmission-type photocathodes, the film thickness is a critical parameter with respect to light absorption and diffusion length. Films of a thickness of one micron or less appear to be most desirable. A typical growth rate for these alloys is about one-half micron per minute so that the entire film is grown in l or 2 minutes or less. FIG. 12 discloses a typical photocathode having a susbtrate 10 of gallium arsenide with a very thin layer of gallium-indium-arsenide 12 disposed upon the surface of the substrate 10.

in the present invention, the doping is controlled by the temperature of the Zn bucket. This temperature can be altered by changing the position of the bucket in the oven. A change in the temperature of the bucket is felt at the growth zone in a few seconds, and the Zn concentration is close to a new equilibrium in about 15 seconds. This corresponds to a thickness of 1,000 A or so. However, this estimate does not consider the time for the temperature of the Zn itself to change when moved to a new point in the zone. A change in temperature from 475 C to 385C results in a change in the doping of from 2 X 10 to 2 X IO Zn/cm. This is enough to give a barrier height of 0.40 eV. Most of this change (0.03 eV) occurs when the doping goes from 2 X 10 to l X 10" which may happen fairly rapidly.

It is interesting to point out that the lower limit to the doping in the bulk for truly high sensitivity is on the order of l X 10", as discussed earlier. Although there is some reduction in the escape probability due to the increased width of the bent band region, the reduction is not large (approx. 30 percent decrease in B). On the high doping side, doping as high as 10 Zn/cm, although it does affect the electron transport in the material, still provides material of good enough quality to yield high sensitivities.

odes selectively responsive in the infrared wavelength range of 1.061;. Quantum efficiencies of from about one to two percent were achieved with these alloys. While the principles of this invention have been described with particularity, it should be understood that various alterations and modifications can be made without departing from the spirit of the invention, the scope of which is defined by the appended claims.

What is claimed is:

l. A photocathode comprising a substrate of gallium arsenide, a thin, continuous, infra-red radiation responsive layer of a gallium-indium-arsenide alloy disposed upon and contiguous with one surface of said' substrate, said alloy layer having the following structural formula:

wherein x is an integer of from about 0.15 to 0.21.

2. A photocathode in accordance with claim 1 wherein x is an integer of from about 0.17 to 0.21.

3. A photocathode in accordance with claim 2 wherein x is the integer 0.205.

4. A photocathode in accordance with claim 1 wherein said alloy layer further includes a P-Type dopant material at a carrier concentration level of from l l0 atoms/cc to 4X10 atoms/cc.

5. A photocathode in accordance with claim 4 wherein said dopant carrier concentration level is 1X10 atoms/cc. 

2. A photocathode in accordance with claim 1 wherein x is an integer of from about 0.17 to 0.21.
 3. A photocathode in accordance with claim 2 wherein x is the integer 0.205.
 4. A photocathode in accordance with claim 1 wherein said alloy layer further includes a P-Type dopant material at a carrier concentration level of from 1 X 1015 atoms/cc to 4 X 1019 atoms/cc.
 5. A photocathode in accordance with claim 4 wherein said dopant carrier concentration level is 1 X 1019 atoms/cc. 